404 - File not Found Problem!

Help Me Please!

I've been having problems with an installation of Windows XP and IIS 5.1. I want to access any file of my IIS from Internet and any try finish in a "404 - File Not Found" on the browser.
I have a PPPoE Connection to Internet with a dynamic IP and the BuildIn Firewall disabled, and a Local Area Network defined.
I have no problem to access with the Local Area Network IP (inside).

Have you allowed port 80 to get through your firewall? In most cases you'll look for port forwarding settings or similar. In a Linksys dsl modem, for example, there is a port forwarding page where you map external traffic to local servers by ports.